A superconducting magnet is an electromagnet whose winding has the property of a superconductor. As in any electromagnet, the magnetic field is generated here by a direct current flowing through the winding wire. But since the current passes in this case not through an ordinary copper conductor, but through a superconductor, then the active losses in such a device will turn out to be extremely small.
Superconductors for magnets of this type are almost always type II superconductors, that is, those in which the dependence of the magnetic induction on the strength of the longitudinal magnetic field is nonlinear.
In order for a superconducting magnet to begin to show its properties, ordinary conditions are not enough - it must be brought to a low temperature, which, in principle, can be achieved in various ways. The classical method is as follows: the device is placed in a Dewar vessel with liquid helium, and the Dewar vessel with liquid helium itself is placed inside another Dewar vessel, with liquid nitrogen, so that the liquid helium would evaporate as little as possible.
As a real example of a powerful superconducting magnet, one can cite the magnet of the Large Hadron Collider (LHC), in which, with the help of a strong magnetic field, it is necessary to keep high-energy protons flying at incredible speed along a certain trajectory inside an extended underground tunnel.
1232 huge electromagnets, each weighing about 30 tons and measuring 15 meters in length, are installed one after another in the LHC tunnel. Proton beams are passed here through thin tubes, and these tubes just pass inside the dipole magnets, the magnitude of the induction of which is adjustable in the range from 0.54 to 8.3 T.
The superconducting properties of magnets at the LHC are achieved by using a special superconducting wire: each magnetic dipole contains an individual superconducting coil wound with a niobium-titanium cable, and the cable itself is made up of the finest wires with a diameter of 6 microns.
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The bottom line is that niobium-titanium is a low-temperature superconductor, so the temperature required to maintain the nominal superconductivity of such windings is only 1.9 K here (lower than the temperature of the background microwave radiation in outer space).
The cooling system of the TANK magnets works thanks to liquid helium, which is in motion all the time. 97 tons of liquid helium are inside a special shell, where superfluidity of this coolant is achieved under a certain pressure.
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Direct cooling of liquid helium occurs under the influence of 10,000 tons of liquid nitrogen. The cooling process is carried out in two stages: a conventional freezing unit first cools the helium to 4.5K, and then it is additionally cooled, but already under reduced pressure. All this action takes about a month in time.
When the temperature conditions are met, it is the turn of the huge currents. At the LHC, the supply current for the magnets reaches 12,000 amperes. At the same time, power is consumed, comparable to that which falls on the electricity supply of the entire city of Geneva. The electrical energy per superconducting magnet is approximately 10 MJ.
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Superconducting magnets are also used in NMR tomographs and spectrometers, in magnetic levitation trains, in thermonuclear reactors, and in many other experimental installations, for example, those associated with levitation .
An interesting fact: weak magnetic fields practically do not have any perceptible effect on diamagnets, but when it comes to strong magnetic fields generated by superconducting magnets, the picture changes significantly. Carbon entering organic objects and living organisms is diamagnetic, so a living frog can hover in a magnetic field with an induction of 16 T.
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